The ancients believed that if you set out a carcass or a pile of dung, after a period of days it would spontaneously create a swarm of flies. I think Aristotle was among the first to stop and actually observe what was going on, and he found flies mating and laying eggs in the usual way of procreation.
I'm no longer certain he was right. For instance, earlier this month when the days were particularly overcast and cold and rainy and icy, and it was difficult to find any invertebrate stirrings of life, I got up after a rainy night (.60 of an inch of rain in my rain gauge) and went to my study in an outbuilding in our backyard. The rain off the roof of the building had left a pool outside the door, and the surface of the water was black with tiny living things. These were springtails, tiny (1 to 2 mm) wingless creatures with six legs that formerly were thought to be primitive insects, but now are thought to be their own kind of six-legged thing. They are, evidently, super-abundant everywhere, on water, on snow, but especially in dirt. If you pick up a handful of soil and examine it closely, you will see tiny little things walking around.
I pointed out this black-covered puddle to Cheryl, and she stood back and looked at the building and showed me that what I had thought was splattered mud on the walls was actually springtails covering every inch of the front of building up to a height of some five feet. They looked like they might have been grazing the algae on the walls. If we were real scientists we might have picked a representative square foot, counted every nearly microscopic dot on it, then extrapolated to the number of square feet that they covered, and finally have gotten something like the actual number present. But since we are "naturalists" rather than "scientists," we were happy to say, oh, hundreds of thousands.
Cheryl took some pictures of them, I made a note in my records. We came out the next morning, and they were gone without a trace.